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Abstract

The study presented in this paper originated from observations made regarding the thermal conditions during winter in highly insulated dwellings with mechanical ventilation with heat recovery (MVHR). Previous observations indicate an oversupply of heat to bedrooms and a successive extensive window ventilation, which leads to an increased space-heating demand.

Detailed simulations were conducted to explain the causes for the observed thermal conditions and to elaborate improved solutions for heating and ventilation during winter. Various MVHR solutions and control strategies, as well as building design solutions, were investigated regarding their impact on the thermal conditions in bedrooms and on the space-heating demand.

The results clearly illustrates that the supply-air temperature and the temperatures in the living room and bathroom have substantial effects on the thermal conditions in the bedrooms. A one-zone MVHR solution, with approximately the same the supply-air temperature to all rooms, has clear limitations regarding the provision of thermal comfort in bedrooms.

The clear potential of a two-zone MVHR solution, where the supply-air temperature to the bedrooms is controlled independently from other rooms, was observed. With a two-zone MVHR solution, the thermal conditions in bedrooms can be improved and the space-heating demand can be reduced.

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Abstract

Efficient energy recovery from used air with the goal of reducing energy use is important for realizing low energy houses. Rotary heat exchangers are very energy efficient, but have the drawback of transferring odours from exhaust air to fresh supply air. To avoid this, flat plate heat exchangers are commonly used where odour transfer might cause problems. Nevertheless, these may not properly handle water condensation and frost formation at low outdoor temperatures. The so-called membrane-based energy exchangers are an alternative to the flat plate heat exchanger. In a membrane-based exchanger, moisture is transferred from the humid exhaust air to the dry supply air avoiding condensation at the exhaust airside. In this work, a membrane energy exchanger was compared to a thin non-vapour permeable plastic foil heat exchanger. The study focused on verifying condensation and freezing problems and evaluating the performance of the membrane energy exchanger. The experiments showed that non-permeable heat exchangers have problems with condensation and freezing under test conditions. Under the same conditions, the membrane-based exchanger did not experience the same problems. However, additional problems with swelling of the membrane in high humidity conditions showed that the tested membrane type had drawbacks and needs further development to become commercially applicable.

Authors:

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,

,

,

Abstract

Efficient energy recovery from used air with the goal of reducing energy use is important for realizing low energy houses. Rotary heat exchangers are very energy efficient, but have the drawback of transferring odours from exhaust air to fresh supply air. To avoid this, flat plate heat exchangers are commonly used where odour transfer might cause problems. Nevertheless, these may not properly handle water condensation and frost formation at low outdoor temperatures. The so-called membrane-based energy exchangers are an alternative to the flat plate heat exchanger. In a membrane-based exchanger, moisture is transferred from the humid exhaust air to the dry supply air avoiding condensation at the exhaust airside. In this work, a membrane energy exchanger was compared to a thin non-vapour permeable plastic foil heat exchanger. The study focused on verifying condensation and freezing problems and evaluating the performance of the membrane energy exchanger. The experiments showed that non-permeable heat exchangers have problems with condensation and freezing under test conditions. Under the same conditions, the membrane-based exchanger did not experience the same problems. However, additional problems with swelling of the membrane in high humidity conditions showed that the tested membrane type had drawbacks and needs further development to become commercially applicable.

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Abstract

A frost-free membrane energy exchanger design model is developed combining the conventional ε−NTU method with a frost limit model. A concept of plate performance index is defined to evaluate the net energy saving ability. The frost-free design model and plate performance index are employed for a case study of single-family dwelling with an all-fresh-air air handling unit with a heat/energy recovery exchanger. The membrane energy exchanger, which is able to ensure frost-free operation without extra frost control strategies, is applicable to most cold climates for residential applications. The membrane energy exchanger has a significant energy saving potential compared to conventional plate heat exchangers. Preheating rather than enlarging the energy transfer area is recommended for severe cold climates.

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Abstract

In highly-insulated buildings such as passive houses, the space-heating distribution subsystem can be simplified by reducing the number of heat emitters. In this context, the bi-directional flow through open doorways is known to be an efficient process to support the heat distribution between rooms. This process is therefore investigated using field measurements within a Norwegian passive house. The so-called large opening approximation proves to model fairly the mass flow rate, but also the convective heat transfer if the thermal stratification is accounted for. Furthermore, the discharge coefficient appears to be independent of the heater type and location in the room.

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Abstract

New buildings have to satisfy ever-tightening standards regarding energy efficiency and consumption. This results in higher insulation levels and lower air leakages that reduce heating demands. However, even at moderate outdoor temperatures these buildings are easily warmed up to such a degree that in order to ensure acceptable indoor environment quality, removal of excess heat becomes unavoidable. Use of electric energy related to mechanical cooling is considered incompatible with achieving zero energy buildings (ZEB). The use of ventilative cooling (VC) in combination with mechanical cooling means energy consumption reduction due to lower use of mechanical ventilation and cooling system.This paper examines the application of ventilative cooling solutions in cold climates through simulations of an existing detached single family house in Norway, the ZEB Living Lab at NTNU/SINTEF. The house has computer controlled motorized windows. This will enable natural ventilation in some part of the year and could then reduce the energy use of fan power. The openable window are placed at the north and south facades and this enables considerably cross ventilation and also stack ventilation as some windows are placed four meters high.IDA ICE program will be used to calculate the energy consumption of the baseline simulation: demand controlled ventilation with variable air volume and mechanical cooling. By means of using CONTAMW the airflow profiles while using controlled window opening are calculated and used as input profiles in IDA ICE to calculate the energy consumption while using hybrid mode ventilation.Results show significant energy savings when using ventilative cooling. Due to the low outdoor temperatures in Norway the use of ventilative cooling remove mechanical cooling demands almost completely. The reference for comparison has been the European standard EN15251 (class II).Ventilative cooling is proven to be relevant in combination with mechanical ventilation and will be crucial to achieving energy targets for new zero energy buildings while the indoor climate is maintained.